1. Field of the Invention
The present invention relates to a semiconductor device in which a MOS transistor is formed on a semiconductor film on an insulating film and, more particularly, to a semiconductor device in which a MOS transistor constituting a sensor amplifier or a boosting circuit of a DRAM is improved.
2. Description of the Related Art
High performance of a recent semiconductor integrated circuit in, especially, a silicon MOS transistor technique is considerably developed. In this silicon MOS transistor technique, it is known that a micropatterned, high-speed, and high-performance element can be realized by forming a MOS transistor on an SOI (Silicon-On-Insulator) film (to be referred to as an SOI-MOSFET hereinafter).
FIG. 1 is a plan view showing a layout pattern of a conventional sense amplifier using such an SOI.multidot.MOSFET, and FIGS. 2A and 2B are sectional views showing the conventional sense amplifiers along lines 2A--2A and 2B--2B in FIG. 1, respectively.
FIG. 1 shows a sense amplifier SA, a bit line BL, a control line 1 for connecting a common source terminal of the sense amplifier SA, a source-contact portion 2, a drain-contact portion 3, and a gate-contact portion 4.
FIGS. 2A and 2B show a p-type monocrystal silicon film 7 as an SOI film. A silicon oxide film (SiO.sub.2 film) 6 is formed on the bottom and side surfaces of the p-type monocrystal silicon film 7. The silicon oxide film 6 on the bottom surface is an insulating film of an SOI substrate, and the silicon oxide film 6 on each side surface is an element isolation insulation film.
An n-type source region 8 and an n-type drain region 9 are selectively formed in the p-type monocrystal silicon film 7. A gate electrode 11 is arranged on the p-type monocrystal silicon film 7 in a channel region between the n-type source region 8 and the n-type drain region 9 through a gate oxide film 10.
In the SOI.multidot.MOSFET, due to a so-called substrate floating effect, problems such as a low drain breakdown voltage or an unstable drain current in a switching operation are posed.
In particular, in a flip-flop type sense amplifier used in a DRAM or the like or a current mirror type differential amplifier, when an n-type SOI.multidot.MOSFET is used in a potential difference detection unit, holes are stored in an SOI.multidot.MOSFET channel portion, and the threshold value of the SOI.multidot.MOSFET decreases. Since the decrease in threshold value depends on the number of stored holes, the decrease in threshold value depends on a transistor. For this reason, the threshold value is unbalanced, and detection sensitivity to a potential difference. When the decrease in threshold value is considerably large, an erroneous operation may be caused.
In a pump circuit constituting a boosting circuit or a lowering circuit, when a capacitor constituting a pump has first and second electrodes, and an SOI.multidot.MOSFET is used as a switching means for connecting the first electrode of the capacitor to an output, a decrease in drain breakdown voltage is caused by the substrate floating effect of the SOI.multidot.MOSFET.
For example, in the lowering circuit, when the first potential is boosted at a timing at which the potential of the second electrode is charged by a is capacitor driver circuit, the SOI.multidot.MOSFET must be turned off. When an n-type SOI.multidot.MOSFET is used the above SOI.multidot.MOSFET, the potential of the first electrode serving as a drain is boosted, the capacity coupling between the drain and the substrate portion of the SOI.multidot.MOSFET boosts the potential of this substrate portion, and the cut-off characteristics of the SOI.multidot.MOSFET are degraded. In the worst case, drain breakdown is caused. In addition, holes generated by slight drain breakdown are stored for a reason except for the above capacity coupling, and drain breakdown is caused by the substrate floating effect.
Furthermore, although an accurate reference voltage generation circuit is required to use the reference voltage as a reference for checking whether an input signal is set to be "H" or "L" in a DRAM or the like, a bulk type pn diode used in a conventional DRAM or the like cannot be used in the SOI.multidot.MOSFET without increasing the number of steps (costs). Therefore, means for generating a stable reference potential without largely increasing the number of steps is desired.
As described above, since no contact with the substrate can be obtained in the semiconductor device using the conventional SOI.multidot.MOSFET, a substrate floating effect is disadvantageously caused. In particular, in sense amplifiers for amplifying a fine potential read out on a bit line pair, since the substrate potentials of two transistors constituting a pair of sense amplifiers are set in a floating state, the threshold values of the transistors are difference from each other, and an accurate sensing operation cannot be performed (subject matter 1). In addition to realization of the accurate sensing operation, high-density integration (subject matter 2), moderation of design rules (subject matter 3), and a countermeasure against noise (subject matter 4) must be realized.
Furthermore, in a pump circuit constituting a boosting circuit or a lowering circuit, the cut-off characteristics of an SOI.multidot.MOSFET are degraded, and drain breakdown is caused in the worst case (subject matter 5). In the SOI.multidot.MOSFET, a stable reference potential cannot be generated without largely increasing the number of steps (subject matter 6).